1. Field of the Invention
The present invention relates to a diffraction element and an optical pick-up assembly utilizing the diffraction element for recording or reproducing information on or from an optical disc.
2. Description of the Prior Art
An example of prior art optical pick-up assemblies is disclosed in, for example, Japanese Laid-open Patent Publication No. 1-237935, published in 1989. This prior art optical pick-up assembly disclosed therein will now be discussed in detail with reference to FIGS. 23 to 25 which are a schematic perspective view, a side sectional view and a bottom plan view of the prior art optical pick-up device, respectively.
Referring first to FIG. 23, a laser beam 12 emitted from a light source 11 passes through a diffraction element 13 and then through a holographic grating (or a hologram) 16 mounted on the diffraction grating 13 in contact therewith. The laser beam 12 is, as it passes through the diffraction grating 13, divided into a zero-order beam 15a, used for reading pit signals on an optical disc 4 and also for sensing a focus deviation, and a pair of first-order beams 15b and 15c used for sensing a tracking error.
The three beams 15a, 15b and 15c emerging outwardly from the diffraction grating 13 are, after having passed through the holographic grating 16, collimated by a collimator lens-17 and are subsequently passed through an objective lens 18. The collimated beams 15a, 15b and 15c are, as they emerge outwardly from the objective lens 18, converged onto the optical disc 14 so as to form respective light spots 19a, 19b and 19c. At this time, each of the light spots 19b and 19c has become a first-order light spot.
The laser beams so projected onto the optical disc 14 are reflected by an information recording surface of the optical disc 14 so as to travel backwards along the same path through which the laser beams have been projected onto the optical disc 14. The reflected laser beams subsequently enter the holographic grating 16 through the collimator lens 17. The laser beams incident on the holographic grating 16 are, after having been diffracted, received by a light receiving element 10', for example, a photodetector, which detects a focus error signal, according to a wedge-prism method, and both of a tracking error signal and an RF signal, represented by pit signals on the optical disc 14, according to a three-beam method.
As shown in FIGS. 24 and 25, the light source 11 and the light receiving element 10' are fixedly mounted on a support disc 20 in a spaced relationship with each other. The support disc 20 has a lower portion forming a radially outwardly protruding flange 20a onto which a lower end of an inner cylindrical barrel 21 is fixedly mounted. The inner cylindrical barrel 21 has an upper end portion remote from the support disc 20 which forms a radially inwardly protruding mount 21a. An assembly of the holographic grating 16 positioned on the diffraction grating 13 in contact therewith is fixedly supported by the mount 21a in a concentric relationship with the light source 11 on the support disc 20.
An assembly of the inner cylindrical barrel 21 together with the diffraction grating 13, the holographic grating 16 and the support disc 20 having the light source 11 and the light receiving element 10' mounted thereon is hereinafter referred to as a projector-sensor module 22. The projector-sensor module 22 referred to above is housed within an outer barrel 23 which will now be described.
The outer barrel 23 has an upper to integral end forming a radially outwardly protruding flange 23a and also has a lower end forming a circumferential row of a plurality of internally threaded holes 23b.sub.1 and 13b.sub.2 each extending inwardly from an annular end face of the bar in a direction parallel to the longitudinal axis of the outer barrel 23. This outer barrel 23 has an internal cavity including a reduced diameter portion adjacent the upper end thereof and a large diameter portion 23d adjacent the lower end thereof. The reduced diameter portion of the internal cavity of the outer barrel 23 defines a lens mount 23c for the support of the collimator lens 17 in alignment with an optical path L through which the laser beam travels, whereas the large diameter portion 23d has an inner diameter greater than the outer diameter of the inner cylindrical barrel 21.
A lower end of the internal cavity of the outer barrel 23 opening downwardly as viewed in FIG. 25 is radially outwardly enlarged at 23e to a diameter substantially equal to the diameter of the radially outwardly protruding flange 20a of the support disc 20. The projector-sensor module 22 is housed within the large diameter portion 23d with the radially outwardly protruding flange 20 seated within the radially outwardly enlarged portion 23e. It is to be noted that the radially outwardly enlarged portion 23e is delimited by an annular shoulder 23e.sub.1 and a side wall 23e.sub.2 lying perpendicular to the annular shoulder 23e.sub.1.
The projector-sensor module 22 so housed within the outer barrel 23 is fixed in position by means of a generally rectangular leaf spring 24 of a configuration which will now be described.
The generally rectangular leaf spring 24 has an intermediate portion 24a which is depressed relative to the opposite ends thereof. This intermediate portion 24a of the leaf spring 24 is of a length sufficient to accommodate therein the maximum diameter of the support disc 20, that is, the diameter of the radially protruding flange 20a and has a rectangular opening 24d defined therein for facilitating the passage therethrough of electric connection lines led from the light receiving element 10'. The opposite ends 24b of the leaf spring 24 has respective holes 24c.sub.1 and 24c.sub.2 defined therein for receiving associated set screws S therethrough as will be described later.
This leaf spring 24 is firmly secured to the lower end of the outer barrel 23 with the set screws S passing through the holes 24c.sub.1 and 24c.sub.2 and then threaded into the internally threaded holes 23b.sub.1 and 23b.sub.2, respectively. At this time, the projector-sensor module 22 is housed within the large diameter portion 23d of the cavity in the outer barrel 23 with the radially outwardly protruding flange 20a of the support disc 20 clamped movably between the annular shoulder 23e.sub.1 and the intermediate portion 24a of the leaf spring 24 while the leaf spring 24 extends diametrically across the support disc 20 as best shown in FIG. 24.
The support disc 20 forming a part of the projector-sensor module 22 housed within the large diameter portion 23d of the cavity in the outer barrel 23 can be turned about its center together with the inner barrel 21 by applying an external turning force to a portion of the radially outwardly protruding flange 20a exposed to the outside. At this time, an outer peripheral wall of the radially outwardly protruding flange 20a integral with the support disc 20 is slidably guided by the side wall 23e.sub.2 defining a part of the radially outwardly enlarged portion 23e of the cavity in the outer barrel 23.
It is to be noted that, in this prior art optical pick-up assembly shown in FIGS. 24 and 25, the light source 11 is fixedly mounted on the support disc 20 at a position exactly aligned with the axis of rotation of the projector-sensor module 22 represented by the axis of rotation of the support disc 20 and, hence, that of the inner barrel 21, which is in turn aligned with the optical path L through which the laser beam travels.
Mounted atop the outer barrel 23 and carrying the objective lens 18 in alignment with the optical path L is a lens carrier 25. This lens carrier 25 is drivingly coupled with any known lens drive mechanism so that the objective lens 18 can be axially moved towards and away from the optical disc 14 for focusing the laser beam projected onto the optical disc 14.
Thus, it is clear that, as discussed in connection with the optical system shown in FIG. 23, the laser beam emitted from the light source 11 travels along the optical path L so as to be projected onto the optical disc 14 through the diffraction grating 13, the holographic grating 16, the collimator lens 17 and the objective lens 18 to thereby form the three light spots 19a, 19b and 19c on the optical disc 14. It is also clear that the laser beam once projected onto and subsequently reflected from the optical disc 14 travels again along the optical path L past the objective lens 18, the collimator lens 17 and the holographic grating 16 and is then received by the light receiving element 10' after having been diffracted by the holographic grating 16.
The adjustment of the tracking signal can be accomplished if the projector-sensor module 22 is turned about the longitudinal axis of the inner barrel 21 which is in turn aligned with the optical path L. The turning of the projector-sensor module 22 results in a corresponding turning of the diffraction grating 13 together with the inner barrel 21 and, therefore, the first-order light beams 15b and 15c used to detect the tracking error, that is, the light spots 19b and 19c on the optical disc 14, undergo angular movement about the light spot 19a. By so doing, the respective positions of the light spots 19b and 19c projected onto the optical disc 14 are adjusted and, consequently, tracking error signals outputted from the light receiving element 10' are adjusted.
As discussed in detail hereinabove, the prior art optical pick-up assembly is carefully assembled so that, in order to avoid an optical misalignment of various component parts at the time of adjustment of the tracking error signal, the light source, the light receiving element and the diffraction grating are integrated together with the inner barrel to form the projector-sensor module. In order to effectively avoid the possible optical misalignment, the various component parts such as the light source, the light receiving element and the diffraction grating are required to be precisely and accurately positioned relative to the longitudinal axis of the inner barrel.
More specifically, according to the prior art optical pick-up assembly, not only must the position of the light source be carefully chosen so as to align with the longitudinal axis of the inner barrel precisely and accurately, but any one of the light receiving element and the diffraction grating must also carefully be positioned at required respective locations relative to the carefully chosen position of the light source, requiring a complicated and time-consuming procedure.